Symbolic Relationship
Broadly defined, symbiosis is the living together of dissimilar organisms, regardless of the possibly injurious or beneficial interactions. In our present discussion, the microbes are called symbionts and insects are the hosts.
Symbionts may live parasitically at the host’s expense. If the host’s vitality is impaired, the symbiont is called a pathogen. Agents causing fatal diseases of insects include certain viruses, bacteria, protozoa, fungi, and probably most parasitic helminths.
Some of these pathogens can be used effectively in controlling pest insects. Insect hemocytes defensively phagocytize the smaller agents and encapsulate the larger ones. Insects do not produce specific antibodies against pathogens, but lysozomes in the gut, fat body, and hemolymph can destroy microbes by enzymatic action.
A coevolutionary tendency exists for the hosts and symbionts to become mutually adjusted. Parasites that kill their host also die, whereas those causing less harm to their host survive. Hosts that are susceptible to parasitic damage are selectively eliminated, leaving resistant hosts in the majority. Thus the relationship evolves toward begin parasites and resistant hosts.
Symbionts that cause no harm to their hosts are commensals, such as the fungi Laboulbeniales. Mutualistic symbionts have evolved a reciprocally beneficial relationship with their host. When the symbionts are outside the insect’s body, e.g., the fungus gardens of insects, they are called ectosymbionts.
Endosymbionts are mutualistic microbes sheltered inside the insect’s body. Buchner recognized that certain groups of insects were dependent on symbionts because the insects’ diet lacked essential items. For example, the vascular fluids of both higher plants and vertebrates, and wood are deficient as food for insects.
Phloem-sucking Homoptera obtain carbohydrate in surplus, but the diet is inadequate in protein; all possess symbionts that supply missing nutrients. Predatory Hemiptera lack symbionts but phytophagous Pentatomidae have them Larvae of Dasyhelea flies (Ceratopogonidae) and Nosodendron beetles (Nosodendridae) live in sap and have symbionts.
Vertebrate blood is notably lacking in B vitamins. Phthiraptera, Cimicidae, Triatoma (Reduviidae), Glossina (Muscidae), and crawling ectoparasitic flies are parasites that feed exclusively on blood in all feeding stages and have symbionts. Adult parasites with scavenging larvae, i.e., Siphonaptera and biting flies, lack symbionts. The larvae of these insects obtain B vitamins sufficient for the rest of the life cycle from bacteria- rich food.
Wood-feeding insects depend on ectosymbionts such as the fungus gardens mentioned above or on endosymbionts in the intestine. Scarab grubs contain bacteria; anobiids and certain cerambycids contain yeasts; and certain termites contain protozoa or bacteria. Certain pests of seeds and stored grain have symbionts: Nysius (Lygaeidae), Lasioderma (Anobiidae), Sitophilus (Curculionidae), Rhizopertha (Bostrychidae), Oryzaephilus (Cucujidae), and Coccotrypes (Scolytidae). Only, the Bruchidae apparently lack symbionts.
Symbionts also occur among insects with a variety of other food habits. Many phytophagous weevils harbour bacteria. Bromius, Cassidae and Donacia are chrysomelid beerles whose larvae feed on green plants and have bacterial symbionts; other chrysomelids lack them. Tephritid flies and lagriine tenebrionid beetles feed in fresh or decayed plants.
Microbes commonly gain entrance to the insect’s body through the mouth as a part of the food. It is not surprising to find most endosymbionts still reside in the gut, having evolved the ability to live in the intestinal environment.
They are often transmitted from generation to generation or among individuals simply by ingestion. For example, the cellulose-digesting protozoa and bacteria of termites are passed among the colony members by anal feeding. Insects without overlapping generations infeet the eggs.
Phytophagous Pentatomidae defecate on the eggs and the nymdhs acquire symbionts after hatching. Some insects have special organs for smering bacteria on the egg as it is deposited. The female Donacia covers the egg with a secretion that encloses a mass of bacteria near the larva’s head. On hatching, the larva eats through the mass and ingests the bacteria.
The hosts’s intestine may be modified to accommodate endosymbionts. The hindgut of certain scrab beetle larvae is greatly enlarged as a fermentation chamber containing bacteria and wood particles. Lateral saclike chambers, or caeco, are characteristic of insects with intestinal endosymbionts.
Cranefly larvae may have a large diverticulum on the hindgut. Phytophagous Hemiptera-Heteroptera characteristically has many bacteria-filled caeca along the posterior portion of the hindgut. Malpighian tubules may also serve as bacterial crypts.
